Seyed Ali Ghoreishian Amiri

Permafrost in marine sediments exhibits a lower freezing point and significant unfrozen water content. This paper investigates the role of the soil freezing characteristic curve (SFCC) in permafrost degradation. Three SFCCs, representing thawing–freezing characteristics of soils with varying clay content and salinity, were established based on experiments and existing data. These SFCCs were then applied in numerical analyses to simulate permafrost thawing under various warming scenarios, using measured ground temperatures and permafrost profiles for a site at Longyearbyen in Svalbard (Norway). It is shown that the ground temperature in non-saline permafrost soil increases more rapidly than saline permafrost, due to a greater downward net heat flux to the permafrost in the former case. Conversely, the thawing rate is more pronounced for saline permafrost soil, attributed to its lower freezing point and latent heat consumption. A more nonlinear ice-melting process is observed for permafrost soil with a lower salinity. The temperature rise follows three stages: a constant-rising, a damp-rising, and an accelerated-rising rates. The duration of the damp-rising rate becomes shorter for saline permafrost under a great warming condition. The study underscores the high significance of the soil-freezing characteristic curve for accurate estimations of permafrost degradation.

Clays exhibit complex mechanical behaviour with significant viscous, non-linear and hysteric characteristics, beyond the prediction capacity of the well-known modified Cam Clay (MCC) model. This paper extends the MCC model to address these important limitations. The proposed family of models is constructed entirely within the hyperplasticity framework deduced from thermodynamic extremal principles. More specifically, the previously developed MCC hyper-viscoplastic model based on the isotache concept is extended to incorporate multiple internal variables and to capture recent loading history, hysteresis and smooth response of the material. This is achieved by defining an inelastic free energy and an element that implements a bounding surface within hyperplasticity, resulting in pressure dependency in both reversible and irreversible processes with a unique critical state envelope, and only eight material parameters with a readily measurable viscous parameter. A kinematic hardening in the logistic differential form in stress space is derived that enables the proposed model to function effectively across a wide range of stresses. Based on this kinematic hardening rule, the current stress state acts as an asymptotic attractor for the back or shift stresses whose evolution rates are proportional to their current state.

The thermodynamically based hyperplasticity framework is employed to develop a hyper-viscoplastic constitutive model describing clay's creep and rate-dependent behaviour. The proposed model complies with the concept of the isotache viscosity and the paradigm of the critical state soil mechanics that is the uniqueness of the critical state friction envelope. A versatile force potential or dissipation rate function is presented that provides adjustability of the location of the critical state while securing a unique critical state friction envelope. A non-associated flow rule as an essential property of frictional material is adopted by further development of the force potential. Adequacy of the proposed constitutive model is evaluated through the simulation of the triaxial tests conducted on Hong Kong marine deposits.

This paper presents a fully coupled thermo‐hydro‐mechanical (THM) model which simulates frost heave in fully saturated soils. The model is able to simulate the formation and growth of multiple distinct ice lenses. The basic equations of the system were derived using the continuum theory of mixtures, nonequilibrium thermodynamics, and fracture mechanics, considering skeleton deformation, water flow and heat transport. Central to this model is the coupled transport of mass due to the temperature gradient across the frozen fringe, which acts as the main driving force of the phenomenon. The model is formulated in terms of measurable physical properties and thus no ad hoc parametrization is required. In an ice‐lens‐free state, the system is solved as a continuum using the finite element method (FEM). It is then locally treated as a discontinuous system upon the formation of ice lens, by enriching the elements carrying the embedded ice lens(es) using the extended finite element method (X‐FEM). The accuracy and efficiency of the proposed model has been verified using several laboratory tests on Devon silt samples at different overburden pressures and thermal boundary conditions. Shut‐off pressures have been also estimated and compared with the experimental results.

In this paper, the transport of sub-cooled water across a partially frozen soil matrix (frozen fringe) caused by a temperature difference over the fringe, is described using non-equilibrium thermodynamics. A set of coupled transport equations of heat and mass is presented; implying that, in the frozen fringe, both driving forces of pressure and temperature gradients simultaneously contribute to transport of water and heat. The temperature-gradient-induced water flow is the main source of frost heave phenomenon that feeds the growing ice lens. It is shown that three measurable transport coefficients are adequate to model the process; permeability (also called hydraulic conductivity), thermal conductivity and a cross coupling coefficient that may be named thermodynamic frost heave coefficient. Thus, no ad hoc parameterizations are required. The definition and experimental determination of the transport coefficients are extensively discussed in the paper. The maximum pressure that is needed to stop the growth of an ice lens, called the maximum frost heave pressure, is predicted by the proposed model. Numerical results for corresponding temperature and pressure profiles are computed using available data sets from the literature. Frost heave rates are also computed and compared with the experimental results, and reasonable agreement is achieved.

This paper reviews eight geoacoustic models applied to frozen soils: crystal growth models (grain cementing, grain coating, matrix supporting, and pore filling), the weighted equation (WE) model, Zimmerman and King's model (KT), the Biot‐Gassmann theory modified by Lee (BGTL), and a two‐end member model. We verify the capacity of these models to estimate unfrozen water content (UWC) based on “reference” UWC results and joint P and S wave velocities for different soil types. The satisfactory UWC estimates of saline unconsolidated sand and overconsolidated clay based on V_p data prove that the KT, BGTL, and two‐end member models are capable of modeling “smooth” transitions in the ice crystal growth mode, while they may provide less accurate UWC values when abrupt change of crystallization mode occurs. None of the tested soil types show a single crystallization mode throughout the freezing process, as assumed by individual crystal growth models. V_s‐based UWC estimates are less accurate due to significant but difficult‐to‐estimate influence of effective stress and soil initial cementation. All models, except pore filling and matrix supporting, can match V_s versus V_p measurement results for sands and silts but gradually provide inconsistent estimates with increasing clay content. We conclude that model validation by independent UWC measurements is necessary and that consistency between UWC values estimated from V_s and V_p is insufficient to ensure proper model validation.

In this paper a two stress-state variables model with solid phase stress and cryogenic suction is introduced for simulating the long-term behavior of frozen soils. The solid phase stress is introduced as the combined stress of soil grains and ice. The model is able to represent the behavior of frozen soils in different temperatures and ice contents with a single set of parameters. It is also able to simulate the transition between frozen and unfrozen states. The so-called over-stress method is applied for simulating the elastic-viscoplastic deformation of the material due to variation of solid phase stress, while an elastic-plastic formulation is applied for the suction induced deformation. In unfrozen state, the model becomes a conventional elastic-viscoplastic critical state model. The model is implemented in PLAXIS and results are compared with the available test data and reasonable agreement is achieved.

This paper gives a summary of some of the main findings of the EU founded project “Creep of geomaterials”, CREEP. CREEP was an Industry-Academia Partnerships and Pathways (IAPP) project funded from the 7th Framework Programme (FP7/2007–2013) of the EC under grant agreement PIAG-GA-2011-286397. The project aimed at establishing a consensus in creep modelling within geotechnical engineering. The materials studied were clay, peat and frozen soils (permafrost). Throughout the project, research on material behaviour in laboratory and field studies was combined with numerical studies using existing and newly developed mathematical frameworks. This paper summarises some of the findings in the project, although the focus is on the developments in the field of soft soils and soft clay in particular. The paper presents a unified enhanced soft clay creep model, which takes into account anisotropy, structure and rate dependency of the material. The performance of the model is demonstrated through analysis of the Murro test embankment. In addition, the paper gives an overview of some characteristics for frozen soil and peat. Some of the considerations regarding, e.g. over consolidation ratio for clay with respect to strain rate are very much valid for peat and frozen soil as well.